Dispersants for inorganic binding agents such as cement or gypsum, for example, have long been known. They are used either for the reduction of the viscosity (plastification) of a suspension of the binding agent at a given water-to-binding-agent factor, or for the reduction of the demand for water while preserving the same consistency.
In his book, "Chemical Admixtures for Concrete" (London 1987), M. R. Rixom describes a total of five chemically different groups of dispersants which are known in modern concrete technology: melamine- and naphthalene-formaldehydesulfonic acid resins, lignin sulfonates, hydroxycarboxylic acid salts and hydroxylated polymers on a polysaccharide basis. In addition, German "Offenlegungsschrift" No. 3,144,673 discloses an additional, novel class of dispersants which consists of keton-aldehyde condensation products containing acid groups.
The action of these known dispersants, however, differs. Rixom describes the melamine- and naphthalene-formaldehydesulfonic acid resins generally as "superplasticizer," since they have by far the greatest dispersing action and produce no undesirable secondary effects. The ketone-aldehyde resins described in German "Offenlegungsschrift" No. 3,144,673 are also reckoned among the "superplasticizer" on account of their excellent dispersing properties. Lignin sulfonates are less effective fluidizers and they also have the disadvantage that, even in their purest, sugar-free form, they decidedly retard cement hydration. Hydroxycarboxylic acid salts and hydroxylated polysaccharides have an even greater retarding action than the lignin sulfonates and can be used only in very small proportions whereby they provide limited dispersing properties.
The dispersants are used, as a rule, in binding agent suspensions in which only small amounts of dissolved inorganic and organic salts, such as sodium or calcium chloride, are contained, In this case the dispersants have the good action described.
In the concrete and cementing technology, however, it is sometimes necessary to prepare binding agent suspensions having contents of soluble inorganic and organic salts of as much as 30% of the weight of the cement content. Examples of this are concreting in cold climates, when calcium chloride and calcium nitrate, particularly, are used in concentrations of 2 to 5 weight-percent as hardening accelerators, or concreting with sea water as mixing water in cases in which fresh water is not available. Concreting in mines, too, often makes necessary the use of saturated sodium chloride solution as the mixing water in order to achieve good strength of adherence of the concrete to saliferous rock. Other examples of the use of binding agent suspensions with a content of soluble inorganic and organic salts are to be found in well cements used in oil, gas and water well drilling. To prevent swelling in clayey deposits and to achieve a tight bond between the cement paste and the formation it is necessary to add salt when preparing the slurry. Furthermore, in the case of offshore drilling only salty sea water is available as the mixing water.
The person skilled in the art knows, however, that melamine- and naphthalene-formaldehyde-sulfonic acid resins as well as lignin sulfonates rapidly lose their good dispersing action when the content of the above-mentioned inorganic and organic salts in the binding agent suspension is high (cf., e.g., D. K. Smith "Cementing" (New York 1976, p. 25)). Some of the hydroxycarboxylic acids and hydroxylated polysaccharides in high proportions are effective dispersing agents even in the presence of salts. In practice, however, additives from these two groups of substances are not usable, since they greatly retard cement hydration when used in the large proportions necessary for a good dispersing effect, and thus they are unsuitable for the economical advancement of construction on account of the poor development of strength.
It is therefore the object of the invention to devise a dispersant for the preparation of binding agent suspensions having relatively high contents of soluble inorganic and organic salts, which can be used in economically feasible proportions, and which at the same time will have no undesirable secondary effects with regard, for example, to the development of strength in the binding agent.